This invention relates to a temperature-compensated crystal oscillator (TCXO) and in particular, to such an oscillator having temperature compensating circuit especially adaptable for a reference clock of a communication apparatus which is strictly required to be stable against temperature variation.
A crystal oscillator uses, as a resonator, a piezoelectric crystal element and usually oscillates at a natural resonant frequency of the crystal element. As materials for the crystal element, there have been known quartz, Rochelle salt (potassium sodium tartrate NaKC.sub.4 H.sub.3 O.sub.6), ADP (ammonium diphosphate NH.sub.4 H.sub.2 PO.sub.4), KDP (potassium dihydrogen phosphate KH.sub.2 PO.sub.4) and other piezo electric crystal materials. Since a typical material is quartz, the following description will be made as regards as a quartz oscillator. However, the description can be applied to other oscillator using, as a crystal element, any one of the other piezoelectric crystal materials.
The quartz oscillator is well known in the prior art as an oscillator which is excellent in oscillation frequency stability.
Recently, a typical use of the quartz oscillator is an oscillator for a reference clock in a digital communication apparatus such as a digital cellular telephone or a GPS (Global Positioning System) receiver. In the application, a frequency fluctuation or deviation ratio of an oscillation frequency must be suppressed within the limits of about .+-.2 ppm, at most .+-.3 ppm (parts per million). The deviation ratio is a ratio of a deviation in oscillation frequency to a predetermined oscillation frequency.
The quartz oscillator of the type has an AT-cut quartz resonator as the crystal element. The AT-cut quartz resonator has a frequency-temperature response where the natural resonant frequency varies in response to variation of circumferential or ambient temperature as shown in FIG. 1. The frequency-temperature response will often be referred to as the temperature-frequency variation characteristic or simply be referred to as a temperature characteristic. Specifically, the AT-cut quartz resonator has a natural resonant frequency as a center frequency of oscillation. It is noted here that the oscillation frequency of the quartz oscillator is determined by and substantially equal to the natural resonant frequency of the AT-cut quartz resonator. The natural resonant frequency has a different value in dependence upon an ambient temperature. In particular, the AT-cut quartz resonator has a predetermined natural resonant frequency corresponding to a predetermined temperature (typically, the room temperature). Let the natural resonant frequency and the predetermined natural resonant frequency be represented by f and f0, respectively. A deviation between the natural resonant frequency f and the predetermined natural resonant frequency f0 is represented by .DELTA.f (=f-f0). A frequency deviation ratio is defined as a ratio .DELTA.f/f0 of the deviation .DELTA.f to the predetermined natural resonant frequency f0. Then, the temperature-frequency variation characteristic of the AT-cut quartz resonator can be expressed as a variation of the frequency deviation ratio .DELTA.f/f0 in response to the temperature variation. In FIG. 1, the temperature-frequency variation characteristic is represented by a plurality of cubic curves each of which has a positive cubic coefficient with an inflection point at the predetermined temperature. The respective curves in FIG. 1 correspond to a plurality of quartz resonators cut out at different cutting angles offset from an AT-cut center angle (35.degree. 0'15") by -2' to +16' at every 2' interval.
As seen from FIG. 1, even if the AT-cut quartz resonator has a relatively small temperature dependency, it is extremely difficult to suppress the frequency deviation ratio within a range of .+-.5 ppm as far as no temperature compensation is carried out. As a result, it is difficult to achieve an excellent frequency stability in the quartz oscillator using the AT-cut quartz resonator.
As described in the beginning, in order to use the quartz oscillator as the reference clock in the digital communication apparatus of the type, the frequency deviation ratio must fall within the limits of about .+-.2 ppm, at most .+-.3 ppm, throughout an operation temperature range. It is therefore required to compensate such temperature-dependent frequency variation.
In view of the above, proposal has been made of two types of temperature-compensated quartz oscillators each comprising an AT-cut quartz resonator. A first one is known as a direct compensation type while a second one is referred to as an indirect compensation type.
Referring to FIG. 2, the temperature-compensated quartz oscillator of a direct compensation type comprises an oscillation circuit including the quartz resonator, and a temperature compensating circuit comprising a temperature detecting element such as thermistors (temperature sensitive resistance elements), resistor, and capacitors. The temperature characteristic of the oscillation circuit is controlled by directly utilizing temperature characteristics of the temperature detecting elements.
Generally, most of commercially available temperature-compensated quartz oscillators are of the above-mentioned direct compensation type.
On the other hand, the temperature-compensated quartz oscillator of an indirect compensation type has a temperature detecting element separate from the oscillation circuit. One of load capacitances in the oscillation circuit is used as a temperature compensating element. The oscillation circuit has a known capacitance-frequency variation characteristic. The load capacitance as the temperature compensating element is implemented by a variable-capacitance diode, a varactor diode, or varicap of a voltage-controlled type. The varicap is applied with a varicap control voltage. By varying the varicap control voltage, the load capacitance is controllably changed. Thus, an overall capacitance of the oscillation circuit is changed to thereby control the oscillation frequency of the oscillation circuit.
In the temperature-compensated quartz oscillator of an indirect compensation type, control is typically carried out by digital operation. Such temperature-compensated quartz oscillator is illustrated in FIG. 3.
Referring to FIG. 3, the temperature-compensated quartz oscillator of an indirect compensation type using digital operation comprises a temperature detecting oscillation circuit with a quartz resonator having a frequency-temperature characteristic preliminarily measured, a frequency counter for frequency measurement, an A/D converter, an EPROM, a D/A converter, and an oscillator circuit.
The EPROM stores frequency-temperature characteristic data of the temperature detecting oscillation circuit, and temperature-voltage conversion data for obtaining the varicap control voltage in correspondence to the temperature characteristic of the output oscillation circuit preliminarily measured.
In the temperature-compensated quartz oscillator of an indirect compensation type using digital operation and having the above-mentioned structure, the varicap control voltage is extracted from the oscillation frequency of the temperature detecting oscillation circuit with reference to the data stored in the EPROM. The varicap control voltage is applied to the varicap to adjust the capacitance of the varicap. Thus, the oscillation frequency of the output oscillation circuit is controlled.
The above-described temperature-compensated quartz oscillator using digital operation in temperature compensation is called DTCXO (digital TCXO). For use in ultra-high-accuracy applications such as a reference oscillator in a base station of a digital communication system or in a high-accuracy measurement apparatus, the DTCXO having a frequency deviation ratio not higher than 0.1 ppm is commercially available.
However, each of the temperature-compensated quartz oscillators of the above-mentioned two types has various problems which will hereafter be described.
Specifically, in the temperature-compensated quartz oscillator of a direct compensation type, it is necessary to combine the resistors, the thermistors, and the capacitors for use in the oscillation circuit, very carefully checking their temperature characteristics. Otherwise, the frequency deviation ratio of the oscillation circuit can not be suppressed within the desired limits of about .+-.3 ppm.
However, a high cost is resulted from assembly of those components very carefully checking their temperature characteristics one by one in an actual mass-production process. Sometimes, it is difficult to obtain those components having a desired combination of the temperature characteristics. Thus, reduction in cost and easy mass production are difficult to achieve.
On the other hand, the DTCXO requires two quartz oscillation circuits (the temperature detecting oscillation circuit and the output oscillation circuit) so that the number of the components is increased and that the manufacturing process becomes complicated.
In addition, the temperature-compensated quartz oscillator of an indirect compensation type comprises expensive components such as the EPROM, an IC for digital operation, the A/D converter, and the D/A converter.
Furthermore, as illustrated in FIG. 4, compensation is carried out with respect to discrete or discontinuous temperature points specified in the data stored in the EPROM. Specifically, each temperature point is a central control value of each temperature zone having a temperature step width. Therefore, the temperature-frequency variation characteristic is plotted as shown in FIG. 5. As seen from the figure, discontinuity occurs between adjacent temperature zones.
In order to remove the discontinuity, it is proposed to change the temperature step width of each temperature zone. Specifically, in a temperature zone exhibiting a large variation of the oscillation frequency, the temperature step width is narrowed. On the other hand, in a temperature zone exhibiting a small variation of the oscillation frequency, the temperature step width is widened.
The above-mentioned approach, however, results in further increase in cost. In any event, the problems are still left unsolved.